The largest Arnioceras semicostatum in this piece is 4.5cm, found on Monmouth beach, Lyme Regis. Prepared resting in it's nodule with other Arnioceras surrounding it, when found the edges of two were visible so show some wear. Free standing on a cut base. 
 
Lower Sinemurian, Black Ven Mudstone Member, Charmouth Mudstone formation. 199 MYA
 
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